That's very similar to the problems with databases. It's a large file which gets added to over time. Thus a contiguous space is allocated to it, but once it grows larger a new fragment is created.
Not using torrents, I'm not sure if there's one which would work the following way. Most of the download managers first create a file of the size they're downloading. The content is garble at first, which simply gets filled in as more data is downloaded. This way fragmentation is kept to a minimum in this case. Maybe there's a torrent client which does the same?
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